The dipole antenna is the most fundamental and simple radiating structure, and appears in many physical applications. The basic structure in FIG. 1 is comprised of two collinear wires 1 extending away from each other, sourced by some transient or resonant electrical source 2. The lengths of the wires, which are the arms of the dipoles, are related to the desired radiating frequency. The two-wire structure is well suited for continuous wave applications, or low voltage, low power applications, due to its relatively high impedance geometry of approximately 150 Ohms.
High power radiation is desired for many applications, including electronic disruption. High powers can be achieved from a fat dipole structure, illustrated in FIG. 2, because the larger diameter arms 3 produce high capacitance that decreases the impedance of the structure to tens of Ohms. The application of high voltage resonant energy 4 onto the low impedance structure can generate very high currents that result in large electric field strengths.
Generating high voltage resonant energy and efficiently delivering it onto the dipole structure can be very difficult, since high voltage connections are typically large and result in impedance mismatches with the source. This problem can be mitigated by integrating the resonant source within the dipole geometry.
A number of efforts have been made to radiate high powered RF from dipole antennas. The most common geometry has been with a resonant quarter wave transmission line built into a dipole geometry. Staines describes this method through several patents (U.S. Pat. Nos. 7,215,083 B2, 6,822,394 B2, 7,002,300 B2).
The Staines geometry is essentially a sleeve dipole designed for a specific radiation frequency, and uniquely integrates a switched resonant quarter wave, low impedance transmission line into the dipole geometry, as illustrated in FIG. 3. The Staines device is comprised of a center conductor 5 contained within an outer conductor 6, but electrically insulted by a gas medium. The transmission line section 7 is charged to a high voltage. The switch 8, which is typically a spark gap, closes, causing a resonance to set up on the transmission line, the resonance being related to the length of the complete structure. The center conductor 5 of the transmission line extends beyond the ground section 6 for a distance of a quarter wavelength, such that the length of the complete device is one half wavelength, making the device a half wavelength radiator. The extended center conductor section refers back to the outer conductor in a manner similar to a traditional dipole.
The fat dipole of FIG. 2 has many advantages over the traditional thin wire geometry. The thin wire geometry is characterized by a relatively high impedance, approximately 150Ω, which reduces the current of the device, and thereby reduces the radiated electric field. Conversely, the fat dipole has lower impedance (tens of Ohms), which allows for higher currents in the antenna, resulting in increased radiated electric fields. The thin wire geometry is also a high Q device, requiring well-tuned driver sources 2. The fat dipole is a wider band device, allowing it to be driven by sources 4 that do not exactly match it in frequency and geometry. Lee (U.S. Pat. No. 7,321,341 B2) presents a dipole geometry designed for wideband operation. Lee's geometry uses multiple wires for each dipole arm, with each wire varying in length. As a result, multiple frequencies are simultaneously radiated.
The standard fat dipole has some shortcomings, primarily its signal gain, or radiation efficiency. FIG. 4 illustrates a fat dipole with an external source. The preferred current path 8 is vertical; however, additional current paths 9 propagate through the device, each characterized by a unique path length or wavelength. These additional currents subtract from the total current flow and decrease the radiated electric field, since they do not contribute to the desired radiation frequency. Fat dipole geometries with integrated resonators, as with the Staines geometry, suffer from the same problem. Because the Staines device employs a single switch with a large bodied structure, many current modes result as charge propagates through the line. Staines seems to have somewhat decreased this effect by making his center conductor conical near the switch.
The application of high voltage to a dipole can be made in a wide variety of methods, but is summarily reduced to two basic geometries: 1) a direct connection, and 2) connection via high voltage coaxial cable. High voltage may also be delivered via direct current for a constant voltage, or via pulse charging methods. The pulse charging method can result in much higher radiated electric field strengths if the pulse risetime is shorter than that of the corresponding center frequency of the antenna, so that much higher voltages can be placed on the capacitor before the spark gap closes.